1. Field of the Invention
The present invention relates to a process and a system for detecting an object in relation to a surface, said surface is not necessarily flat. In accordance with the present invention an object is defined as: a material element devoid of life and/or a person or animal. The object in accordance with the invention may be static and/or in movement. The surface in accordance with the invention may be a virtual surface or a real surface transparent or not.
2. Discussion of Background
It is necessary to detect presence and determine the position of an object in relation to a surface in a number of industrial applications, notably in order to make it possible to assess whether the object is moving in relation to this surface. The study of this relative movement gives information on the object and/or the surface.
In order to present the problem posed and resolved by the present invention in the framework of a concrete industrial application, the specific case of electronic surveillance, designed to detect an intruder moving in a defined environment, will be developed hereinafter. These electronic surveillance systems make it possible to distinguish between persons behaving abnormally in relation to those behaving normally.
More specifically, the process and the system according to the invention are applicable to the surveillance of a swimming pool in order to prevent drawings.
The patent SP2 FR 96 10442 submitted on Aug. 26, 1996 on behalf of Poséidon describes this type of surveillance system for a swimming pool while referring to the previous art. Large extracts of it will be incorporated here. Surveillance of swimming pools is either non-existent, or carried out by means of human surveillance. Such surveillance is a difficult task and requires sustained attention and causing nervous fatigue in people, notably in lifeguards, who are responsible for it. In fact, in addition to the inherent limitations of any system based on human intervention, for example a reduction of vigilance due to tiredness or momentary distraction, the surveillance of swimming pools is made extremely delicate because of the reflection of light on the surface of turbulent water, a phenomenon which makes it difficult visually to locate an immobile body or a body sinking passively, at a few metres depth. The surveillance problem in swimming pools mainly arises for swimming pools that are open to the public.
In general, a swimmer in distress sinks to the bottom but, more rarely, it occurs that he floats unconsciously in a characteristic position just below the surface of the water.
An experienced lifeguard has two to three minutes to save a drowning person from the loss of consciousness, which marks the beginning of drowning. If this period is adhered to, the person drowning will not generally experience any after-effects of the accident, after a possible stay in the hospital to clean the lungs. In general, if the help is provided in three to five minutes after the loss of consciousness, however this period varies for each individual, the drowning person may still be saved, but certainly risks irreversible injuries, notably brain damage. Over five minutes, the risk of death becomes considerable.
U.S. Pat. No. 5,043,705 already proposed the use of sonar to carry out surveillance in a swimming pool. According to this system, a sonar transceiver is at least installed in the bottom of the swimming pool and a horizontal section is watched using this machine. The implementation of such a system does present inconveniences. In effect, the signal obtained with a sonar includes the echoes from the swimming pool walls and it is extremely difficult to eliminate the noise signal received in this way in order to detect the signal corresponding to the submerged body of a person drowning. In addition, the sonar essentially makes it possible to locate the body of a drowning person through the volume of air which he holds; if a person in distress has lungs filled with water, the signal obtained will not conform in any way with that which may be expected and might not even be located by processing the signal. Consequently it may be established that such a system is not likely to be satisfactory.
The patent application WO 95/34056 also proposed the implementation of cameras applicable in the field of visible waves for the surveillance of a swimming pool. These cameras are installed so that the zone observed is situated in a volume near and parallel to the bottom of the swimming pool.
In this system the cameras only observe a section of water parallel to the bottom, which implies multiplication of the cameras if the bottom is not flat and leaves the majority of the volume of the swimming pool unwatched. In addition, this system does not make it possible to detect an immobile body floating just below the surface of the water. This system consequently cannot be satisfactory.
A surveillance system for a swimming pool is described in patent application SP2 (FR 96 10442 Poséidon) which consists of:                means of detection (notably video cameras positioned in waterproof compartments) appropriate for giving images of bodies submerged in the water of the swimming pool in the form of electrical signals, these means of detection are planned in the sides of the swimming pool and places judiciously chosen to sweep at least a fraction of the water volume in the swimming pool;        means of digitisation of the electrical signals obtained;        means of temporarily and permanently storing the digital image data at successive moments;        means of comparison between digitised images of the same body at successive moments;        means of estimating the nature of a body (human body or not), of the trajectory and changes in attitude of the body according to these successive images;        and means for making decisions appropriate for setting off an alarm in the event of a suspicious trajectory or movement of the body being observed.        
The means of comparison of the digitised images between successive moments are appropriate only to take forms into consideration, of which the dimensions at least correspond to those of a child, in order to eliminate false alarms being caused by foreign bodies. In addition, these means of comparison are implemented in order to isolate a form and follow its trajectory at successive moments.
The means of estimating are appropriate to determine the slow nature of a movement and/or the immobility of a human body in the swimming pool by means of the results of the means of comparison.
The means for making decisions are appropriate for setting off an alarm if the slow nature of a movement or the almost-immobility of a body in the swimming pool continues over a defined period, in particular over 15 seconds.
One or numerous control screens are positioned by the lifeguards' chairs or in the offices of the persons responsible for surveillance of the swimming pool, screens on which the images are shown of a zone considered suspect. The alarm may be given through a sound and/or visual warning, in particular with an indication of the zone of the swimming pool in which a suspicious event is taking place.
No matter how perfected they are, such systems cannot always make it possible to distinguish an object in front of the bottom. In fact, in the event that a single camera is used, it is not always possible to distinguish a shadow of a body of a swimmer passively floating, moving along the bottom. Admittedly, in the event that numerous cameras are used to observe the same object from numerous points of view, the parallax effect should, in principle, make it possible to distinguish a dense object situated in front of the bottom of a bi-dimensional mark situated on the bottom. In any event numerous conditions must be met in order for the parallax effect to adequately sensitive. On the one hand, it is desirable for the viewpoints to be close to the object being observed. This first condition implies that numerous cameras will be used for surveillance over a large zone in relation to the dimensions of the objects that one hopes to detect. In correlation, this type of system is consequently particularly costly. On the other hand, it is desirable that the depiction of colours perceived by each camera is identical. Now, in order for this to be the case it is essential for the opto-electronic characteristics of the video cameras to be the same, which is not always the case. In addition the optical route between the object and each camera may cross environments with different refraction or transparency indexes. This is notably the case when the body being observed is submerged in a swimming pool with a turbulent surface. The depiction of the colours of the object being observed by each camera is not the same. Consequently the geometric correlations that make it possible to establish that the images (their outlines and grey scale nuances) produced by each camera come from the same dense object situated in front of a coloured bottom, can not longer be verified with certainty. Consequently confusion is possible between a shade of colour (for example a shadow being carried) on the bottom of the swimming pool and a dense object close to the bottom. Consequently, the result is that errors in detection and false initiation of the alarm systems.
The present invention is intended to avoid the inconveniences of the processes and systems known as yet, no matter how efficient they are.